Testing operation of electric energy meter optics system

ABSTRACT

A method and apparatus for testing the proper operation of the optical elements in the optical detection circuit of an electronic register in an energy meter. The light emitter circuit is periodically pulsed adequate to turn the emitters to the on condition, and a comparison circuit is provided in which the voltage across a monitor point in circuit with the light emitters and a switching circuit is monitored both before and after each pulse. The absence of a voltage change upon pulsing the light emitter circuitry is used to generate a first error or failure signal. The light detector is read prior to each pulse to determine if unprogrammed light is being received prior to the pulsing of the light emitter as an indication of tampering with the accurate reading of the energy meter, and a second error signal is generated in response to unprogrammed light. The first error signal is displayed at the energy meter to the meter reader, while the second error tampering signal is stored and downloaded through an optical link for analysis at the power company.

This application is a continuation of application Ser. No. 07/628,133filed Dec. 17, 1990, now abandoned, which is a division of applicationSer. No. 07/505,199 filed Apr. 5, 1990, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to energy meters with electronicregisters, and more particularly to the provision of a method andapparatus for testing the operation of the optical electronics systemwhen used in such meters.

Present-day electronic registers have greatly expanded capabilities andare being called upon to provide additional functions beyond simplemeasurements of kilowatthour energy consumption and kilowatt demand. Theinclusion of microprocessors in electronic registers has provided thecapability to provide a plurality of additional functions, and makeadditional information available to the customer, the meter reader andthe serviceman. In electronic registers, it is well-known to utilizeoptical electronics to measure energy consumed by the load beingmetered. In a combination induction electric watthour meter and time ofuse or demand electronic register, the watthour meter utilizes an eddycurrent disk which rotates in response to the rate of energy consumptionbeing metered. A shutter is commonly mounted on the same shaft as theeddy current disk and is positioned to rotate between a light emitterand a light detector. A plurality of circumferential teeth on therotating shutter sequentially break the light beam between the lightemitter and light detector to provide a series of electrical pulseswhich are directly proportional to the rate of energy consumption by theload being metered.

It is common practice to provide an optical communications link betweenthe electronic register and the outside of the meter in order to providean interface to a portable reader, providing an automatic means forextracting billing data and for programming the electronic register.

It is possible that the electronic circuitry associated with the disksensing optics system may fail even though the eddy current diskcontinues to rotate and the induction watthour meter continues toproperly display kilowatt hours of power consumed by the load beingmetered. A problem exists if a register light emitter should fail,particularly if this occurs shortly before the electronic register isread by the meter reader. For example, if meter readings are taken oncea month and if one of the light emitters fails just prior to aparticular meter reading, the absence of the time of use readingsprovided by the electronic register may not become apparent until thefollowing month or other period when the register is next read. It isthus desirable to provide for the testing of the operation of the lightemission circuitry in the disk sensing optics system of an electronicregister, since this is more likely to fail than the induction watthourmeter. This would provide the meter reader with an indication of theoperation of the disk sensing optics system circuitry each time, and atthe time, the electronic register is read and enable prompt correctiveaction. In particular, it is desirable to be able to determine whetherthe light emitter circuitry is operating at the time of meter reading,while at the same time minimizing the requirement for additionalcomponents and added complexity of the electronic register.

As with all energy meters, including energy meters with electronicregisters, where the customer pays for energy consumption based on themeter readings, it is important to provide the ability to detecttampering attempts which can be anticipated. One form of tamperingincludes the use of high intensity light, particularly infrared light,which is directed from the outside of the energy meter towards the disksensing optics assembly. Since energy consumption, or the rate of energyconsumption, is detected in the disk sensing optics assembly by the rateat which the toothed shutter interrupts the light path between the lightsource and the light detector as it rotates between them, an extremelybright light shining on the light detector could interfere with thedetector sensing interruptions of the normal operating light beam by thepassage of teeth on the toothed shutter.

As a result, means are needed to detect whether there has been tamperingwith the electronic register through use of an extremely bright externallight source. It is also highly desirable that this be accomplishedsimply, reliably, and efficiently.

In addition, it is desirable that the failure error signal be availableat the electronic register for the information and use of the meterreader. It is desirable that the tamper error signal be stored within,but not available at, or displayed by, the electronic register. It isdesirable that the tamper error signal be provided instead to the officeof the power company for analysis and appropriate action, whethercustomer relations or legal action, as appropriate.

OBJECTS AND SUMMARY OF INVENTION

It is an object of the present invention to provide means for testingthe operation of the disk sensing optics system of an electronicregister.

It is a further object of the present invention to provide for thetesting of the light emitter circuitry of a disk sensing optics systemin an electronic register with a minimum of components and complexity.

It is another object of the present invention to provide a means oftesting the disk sensing optics systems of an electronic register forexcessive external light.

It is yet another object of the present invention to detect tamperingwith an electronic register and to store that information for subsequentretrieval without alerting the person doing the tampering, whiledisplaying other error signals at the electronic register.

In accordance with one embodiment of the present invention, a comparatoris provided to test the light transmission circuitry in an electronicregister utilizing the interruption of electro-optical lighttransmission between infrared light emitters and light detectors tomeasure energy use. The infrared emitters are connected in series with aswitching transistor which is pulsed to energize the infrared emitters.The current through the infrared emitters is checked before and duringthe energizing of the infrared emitters. An error is detected if thereis no significant difference.

More particularly, the comparator circuit includes a reference voltageand a transistor connected in series with a resistor and the lightemitters. The difference in current flow through the series circuit as aresult of light detector pulsing is checked by monitoring the voltage atthe junction of the resistor and the light emitters. A voltage divideris connected in series with the resistor and in parallel with the lightemitters and transistor. The voltage across one resistor in the voltagedivider is fed to a comparator whose output is connected to amicroprocessor which stores, and delivers on command, an indication of alight emitter error when the voltage being monitored does not changewith the pulsing of the transistor.

The light detectors are read prior to pulsing of the light emitters todetermine if light is being received without pulsing which wouldindicate possible light tampering by a person attempting to evade properenergy consumption billing by shining a bright light on the detectors.The tamper error signal is stored in the electronic register but is notmade available at the energy meter as is the failure error signal. Thetamper error signal is downloaded through an optical link to thecomputer at the office of the power company for analysis and response.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an electronic energy meter incorporating one embodimentpresent invention.

FIG. 2 is a side view of FIG. 1.

FIG. 3 shows the optical electronic arrangement for obtaining pulsesresponsive to energy consumption.

FIG. 4 is a top view of FIG. 3.

FIG 5 shows the circuitry provided to test the operation of the opticssystem.

FIG. 6 is a block diagram useful in explaining the procedure and methodused in providing an output indication on the status of the lightemitting optics system.

Referring first to FIGS. 1 and 2, an electric energy or power meter 2 isshown in simplified form, and includes a base 4 having a plurality ofmeter circuit terminals such as 8 and 10 extending through the base toconnect the meter in circuit with a power source and a load which is tobe metered. An electronic register assembly 20 is positioned remote fromthe base 4 and meter circuit terminals 8 and 10. The internalelectronics and mechanism of the energy meter 2 are enclosed by atransparent cover or enclosure 22 which is secured to the base 4 by alocking ring 24. In a manner well known in the art, the eddy currentdisk 27 on shaft 30 is caused to rotate within bearings 32 and 34 at aspeed proportional to the electric energy provided to, and consumed by,the load to which the electric energy meter 2 is connected throughcircuit terminals such as 8 and 10, and the watthour meter 5 integratesthe revolutions of the eddy current disk as a measure of powerconsumption. A circular shutter disk 28 is also supported on shaft 30for rotation with eddy current disk 27.

The electric energy meter 2 in addition to the induction watthour meter5 with a dial read out assembly 7, includes an electronic registerassembly 20. The electronic register 20 includes a liquid crystaldisplay 15, and an optical coupler 17 which is part of the opticalcommunications port 29. Details of an electric energy meter to which thepresent invention is applicable are set forth in United States Patentapplication, Ser. No. 07/505,195, issued Jun. 25, 1991, U.S. Pat. No.5,027,056 of A. A. Keturakis, S. D. Velte, J. G. Rusillo, Jr., and R. A.Balch assigned to the same assignee as the present invention, and herebyincorporated by reference.

The optical communications port 29 is provided to optically connect theelectronic register 20 to the outside of the energy meter 2. As shown inFIG. 2, the optical communications port 29 includes a shroud assembly 31between the electronic register assembly 20 and transparent cover 22,and an optical coupler 33 positioned outside, and passing through, thecover to provide an optical connection from outside the cover to theoptical electronic register 20 of the energy meter 2. A meter readerduring periodic readings, such as once a month, positions an opticalreader, shown generally as 35, over the optical coupler to read theoutput of register assembly 20, and if required, to program, orreprogram the register time of use periods. The output of the registerassembly 20 is stored in the optical reader, and subsequently downloadedinto a computer at the office of the power company for billing andanalysis purposes. As shown in FIG. 1 a light emitter 41 and lightdetector 43 positioned within the enclosure 45 of register assembly 20provide the optical interface between the optical reader 35 positionedover the optical coupler 33 and the optical electronic circuitry of theregister assembly. The details of the optical communications port 29 isdescribed in copending U.S patent application Ser. No. 07/505,970,issued Oct. 15, 1991 as U.S. Pat. No. 5,057,767 filed by A. A. Keturakisand S. D. Velte, assigned to the same assignee as the present invention,and hereby incorporated by reference.

As shown somewhat schematically by FIGS. 3 and 4, the shutter 28 ismounted on, and rotates with, shaft 30 between spaced jaws 40 and 42 ofthe disk sensing optics assembly 44. As best shown in FIG. 4, theoptical shutter 28 is castellated and includes a plurality ofcircumferential radially extending teeth 46 which sequentially passbetween the spaced jaws 40 and 42. Positioned within the spaced jaws 40and 42 are 2 light emitters 84 and 86, and paired detectors 88 and 90,respectively, which provide light transmission and reception in the pathof the teeth 46. Upon rotation of the disk 28 in the direction shown,for example, by arrow 48, the teeth 46 will sequentially rotate firstpast one light emitter and detector pair 86, 90 and then past the secondlight emitter and detector pair 84, 88. A light source or light emitter84, 86 is thus positioned above and on the opposite side of the opticalshutter 28 from the cooperating light detectors 88, 90 respectively. Asshown in FIG. 3, the light detector 88 is positioned in jaw 42 below alight emitter 84 in jaw 40. Light detector 90 in jaw 42 is similarlypositioned below a light emitter 86 in jaw 42, and on the opposite sideof the optical shutter 28, such that the light transmissions from thelight emitters to the light detectors are interrupted every time a tooth46 passes between the jaws. The light emitters 84 and 86 may be lightemitting diodes (LEDs) providing infrared (IR) light upon energization,and the light detectors 88 and 90 are responsive to the type of lighttransmitted by the light emitters.

The disk sensing optics assembly 44 of the present invention is securedto the register printed circuit board 62 which is located within theelectronic register assembly 20 of the electric energy meter 2 asdisclosed in copending patent application Ser. No. 07/505,383, issuedJul. 23, 1991 as U.S. Pat. No. 5,034,682 of A. Keturakis, R. C. Mayo,and S. D. Velte, assigned to the same assignee as the present inventionand hereby incorporated by reference.

Referring next to FIG. 5, FIG. 5 is a schematic of a comparator circuit100 used to provide signals which are indicative of the operation, or ofthe failure, of a light emitter 84 or light emitter 86. Referring toFIG. 5, the light emitters 84 and 86 are connected in series withresistor 102 and the collector 160 of transistor 104. The emitter 161 oftransistor 104 is connected to ground 108, and the remote end ofresistor 102 is connected to the supply voltage 106 which varies from 10to approximately 16 volts. A voltage divider 110 consisting of resistors114 and 116 is connected between ground 108 and the monitor point 112 atthe junction between resistor 102 and light emitter 84. The junction 120between resistors 114 and 116 is connected to the positive terminal ofthe comparator 118, while the negative terminal of comparator 118 isconnected to a reference voltage 122 of 2.7 volts. The output 132 ofcomparator 118 is provided to microprocessor 124 of the electronicregister 20.

Current flow through the resistive circuit of 102, 114 and 116, causes avoltage at monitor point 112 Which is higher than the reference voltage122. Current flow through the light emitters 84 and 86 upon pulsing oftransistor 104 to the on condition causes additional current flowthrough resistor 102, such that the voltage at monitor point 112 willdecrease to approximately 4 volts.

The voltage divider consisting of resistors 114 and 116 is connected inparallel with, and across the voltage at monitor point 112, such thatthere is current flow through the series circuit consisting of resistors102, 114 and 116 whether or not there is current through the parallelbranch including light emitters 84 and 86, transistor 104, and resistor102. The resistance of the voltage divider 110 is large compared to theresistance of the branch including light emitters 84 and 86 andtransistor 104 when there is conduction through the light emitters.Accordingly, current flow through the branch consisting of the lightemitters 84 and 86 and transistor 104 will significantly increase thecurrent flow through resistor 102, significantly lowering the voltage atmonitor point 112 in the presence of such current flow.

The signal at junction 120 of voltage divider 110 is thus responsive tothe voltage at monitor point 112 and varies in accordance with currentflow through the branch including the light emitting diodes 84 and 86and the transistor 104. When such current flow occurs, the voltage atjunction 120 is low, or in the low state. In the absence of current flowthrough the light emitting diodes 84 and 86 and transistor 104, thevoltage at junction 120 is high, or in the high state.

Operation of the present invention can best be understood with referenceto FIGS. 3-6 and in particular to FIG. 6. Referring to FIGS. 3-6, thestate of the optics is first checked by reading the output of the lightdetectors 88 and 90 which will be in the high state when the lightemitters 84, 86 are off.

During every period of the basic interval interrupt of 7.8125milliseconds, the microprocessor 124 timing circuit provides a timingsignal 126, a series of 150 microsecond positive pulses or square wavessuch as 138 and 140, to the base of transistor 104 through a seconddivider circuit 142 consisting of resistors 144 and 146. Light emitters84 and 86 are thus periodically pulsed for approximately 150microseconds, that is, for a relatively small portion of the short basicinterval interrupt. The pulses cause conduction of transistor 104through resistor 102, and the light emitters 84 and 86, only if thelight emitters are operating. If the light emitters 84 and 86 areconducting, the voltage at the monitor point 112 will decrease, which isreflected at junction 120, providing negative pulses 148 and 150 ofapproximately 150 microseconds through comparator 118 to themicroprocessor 124 indicating that the light emitting diodes areoperating.

If a light emitter 84 or 86 fails, there will be no light transmissionby the light emitters in response to pulses such as 38 and 140 and thevoltage at monitor point 112 will remain in the high state. Thesubstantially constant signal provided to the microprocessor 124 throughcomparator 118 indicates a light emitter 84, 86 error. This lightemitter error signal or light transmission error signal 153 is stored inthe microprocessor 124 and displayed to the mete reader at liquidcrystal display 15 during reading of the energy meter 2.

Thus, the comparator circuit 100 is used to monitor the voltage drop at112 caused by the conduction and emission of the two light emitters 84and 86 and the transistor 104 as an indication of operation of the lightemitters.

However, in checking the optical electronics system of an electronicregister 20, it is desirable that certain of the errors or faults suchas a failure error signal be displayed at liquid crystal display 15 ofthe energy meter 2 for the meter reader's use and information, whilecertain other errors such as a tampering error signal be stored withinthe energy meter and displayed, interpreted and responded to by otherswithin the power company. Errors which indicate possible illegaltampering with the energy meter readout 2 for purposes of evadingbilling charges should be analyzed and acted upon by personnel otherthan the meter reader, possibly including legal and law enforcementpersonnel. Accordingly, the tampering error signal indicating possibletampering is detected, stored and subsequently provided and displayedaway from the energy meter 2, and to personnel other than the meterreader, or the person tampering with the meter.

Since the time of use energy demand readings are used for billingpurposes, it is possible as pointed out above that tampering, orattempted tampering, with the operation of the energy meter will beencountered. The optical electronics system of electronic register 20utilizes the counting of pulses obtained when the light beam from thelight emitters 84, 86 are disrupted by the rotating toothed opticaldisk. Tampering with such systems have included directing an extremelybright light into the enclosure 45 to provide continuous light to thelight detectors, which may, as a result, be unable to detect disruptionsin light transmission from the light emitters 84, 86 by the teeth 46 inthe optical assembly 44. Accordingly, it becomes important to detectsuch light tampering.

As a result, the condition of the light detectors 84 and 86 is readbefore the pulsing of the light emitters 84 and 86 to determine if theyare receiving light. Under such conditions, the presence of spuriouslight would be an indication of possible tampering, and a lighttampering error signal 155 is generated. It is desirable that the lighttampering error signal 155 be made available only to skilled personnelat the office of the power company for interpretation and correctiveaction. Such corrective action might involve seeking additionalconclusive proof for possible legal or other action. For example,unscheduled readings or observations could be undertaken in cooperationwith legal authorities to attempt to observe and establish theunauthorized light tampering. Accordingly, the light tampering errorsignal 155 is stored in microprocessor 122 and downloaded through anoptical reader 135 to the computer at the office of the power companyfor analysis and corrective action.

While the present invention has been described through preferredembodiments, such embodiments are provided by way of example only.Numerous variations, changes and substitutions, including thosediscussed above will occur to those skilled in the art without departingfrom the scope of the present invention in the following claims.

What I claim is:
 1. Apparatus, comprising:metering means configured to be coupled between an end user and a power delivery system, said metering means comprising a rotatable shaft having shutter means attached thereto, at least one emitter/detector pair means disposed for having a portion of said shutter means pass therethrough and for generating electrical pulses indicative of a quantum of energy consumed by the end user, means for energizing said emitter means during a basic interval interrupt, the duration of the basic interval interrupt being in excess of the duration of energizing said emitter means, said emitter means having an on condition and an off condition and transitioning from the off condition to the on condition when energized, the emitter means being in a conducting condition and emitting a first signal when in the on condition; means for checking whether said emitter means responds to a pre-determined manner as a result of energizing said emitter means, said checking means comprising means for providing the signal from a monitor point of said emitter means to comparator means for determining whether the signal from the monitor point is within a predetermined range, said emitter means and switching means being connected in series with an impedance, and wherein the signal provided from the monitor point is representative of the voltage at one end of the impedance; and means for generating an error signal when the emitter means does not respond to the energization in the pre-determined manner, the error signal being generated when the voltage at the monitor point does not change in a pre-determined manner as a result of energizing said emitter means.
 2. An apparatus in accordance with claim 1 wherein the emitter means comprises means for emitting a first light signal.
 3. An apparatus in accordance with claim 2 wherein the emitter means comprises a light emitting diode.
 4. An apparatus in accordance with claim 2 wherein the checking means further comprises means for detecting the presence the first light signal.
 5. An apparatus in accordance with claim 1 wherein said energizing means comprises switching means and energy supply means.
 6. An apparatus in accordance with claim 1 further comprising storage means for storing the error signal.
 7. A method of operating an electric energy meter in which energy consumption is measured utilizing a member rotatable between one or more light emitter and light detector pairs, said method comprising the steps of:energizing the light emitter circuitry, the light emitter circuitry being energized during a basic interval interrupt and the duration of the basic interval interrupt being in excess of the duration of energizing the light emitter circuitry; checking whether the light emitter circuitry responds in a pre-determined manner as a result of energizing the light emitter circuitry, said checking comprising the step of providing the signal from a monitor point of the light emitter circuitry to comparator means for determining whether the signal from the monitor point is within a predetermined range, said one or more light emitters and switching means being connected in series with an impedance, and wherein the signal provided from the monitor point is representative of the voltage at one end of the impedance; and generating an error signal of the light emitter circuitry does not respond to the energization in the pre-determined manner, the error signal being generated when the voltage at the monitor point does not change in a pre-determined manner as a result of energizing the light emitter circuitry.
 8. A method in accordance with claim 7 wherein energizing the light emitter circuitry comprises the step of providing one or more energy pules to the light emitter circuitry and during a basic interval interrupt to initiate the normal operation of said light emitter circuitry during the basic interval interrupt.
 9. A method in accordance with claim 1 wherein the light emitter circuitry is energized by one or more energy pulses that are substantially square waves, and the light emitter circuitry is pulsed for a period having a duration of approximately 150 microseconds.
 10. A method in accordance with claim 9 wherein the basic interval interrupt has a duration of approximately 7 milliseconds.
 11. A method in accordance with claim 10 wherein microprocessor means coupled to the comparator means generates the error signal as indicative of a failure of the light emitter circuitry, the microprocessor means generates the error signal when the comparator means provides a voltage signal without significant change upon application of the one or more pulses to the light emitter circuit during the basic interval interrupt.
 12. In an electronic register for an electric energy meter in which energy demand is measured through a toothed optical shutter rotating between one or more emitter and detector pairs, a method of checking operation of the emitter circuit comprising the steps of:connecting the light emitters in circuit with a switch which is pulsed to provide conduction through said emitters when no error exists; monitoring a voltage within the emitter circuit before providing pulses to said switch; monitoring said voltage within said emitter circuit after providing one or more pulses to said switch which are adequate to turn said emitters to the on condition; determining whether there has been substantial change in the voltage being monitored before and after said pulses; providing a light transmission error signal when there is no substantial change in said voltage being monitored upon the provision of said one or more pulses; and displaying said light transmission error signal at said electronic register.
 13. The method to check operation of the light emitter circuitry in an electronic register of claim 12 wherein monitoring of said voltage is done during the period for energization of said one or more emitters.
 14. The method to check operation of the emitter circuitry in an electronic register of claim 13 wherein a microprocessor is connected in circuit with said light transmission error signal, storing said light transmission error signal in said microprocessor, and subsequently displaying said light transmission error signal at said energy meter upon a command signal to said microprocessor.
 15. The method to check operation of the emitter circuitry in an electronic register of claim 14 wherein said one or more pulses are provided for a duration of time significantly less than the duration of the periodic basic interval interrupt.
 16. The method to check operation of the emitter circuitry in an electronic register of claim 15, wherein a timing circuit provides a signal to measure the monitored voltage at the end of each of said one or more pulses.
 17. In an electronic register for an electric energy meter in which energy demand is sensed through a discontinuous shutter rotating between and breaking the light transmission between one or more emitter and detector pairs, a tester for checking the failure of one or more of the one or more emitters and light transmission comprising;one or more emitters in circuit with switching means and a resistive impedance across a supply voltage; means to pulse said emitter circuit to energize said one or more emitters under normal conditions; the pulses to said emitter circuit being of a smaller time duration than the time duration of the periodic basic interval interrupt of said electronic register; a comparator responsive to the failure of one or more of said one or more emitters; said comparator connected to a monitor point in circuit with said one or more emitters; and said comparator comparing the signal level at said monitor point during the basic interval interrupt period before and after said one or more pulses, and providing an emitter error signal when the signal level, before and after said one or more pulses is substantially the same.
 18. A light emitter circuit tester in an electronic register for an electric energy meter of claim 17 wherein storage means are provided wherein said emitter error signal is stored and later provided upon the provision of a command signal.
 19. A light emitter circuit tester in an electronic register for an electric energy meter of claim 17 wherein said comparator includes a reference voltage.
 20. A light emitter circuit tester in an electronic register for an electric energy meter of claim 19 wherein a voltage divider is connected to said monitor point and across said one or more light emitters.
 21. A light emitter circuit tester in an electronic register for an electric energy meter of claim 20 wherein the emitter error signal is developed at a junction of said voltage divider, said junction being connected to a microprocessor for storage of said emitter error signal, and subsequent delivery of said emitter error signal upon the presentation of a command signal to said microprocessor.
 22. A light emitter circuit tester in an electronic register for an electric energy meter of claim 21 wherein said one or more emitters are light emitting diodes.
 23. A light emitter circuit tester in an electronic register for an electric energy meter of claim 22 wherein said means to pulse said emitter circuit includes one or more transistors.
 24. A light emitter circuit tester in an electronic register for an electric energy meter of claim 23 wherein each of said one or more pulses are in the order of approximately 150 microseconds duration, and said periodic basic interval interrupts are in the order of 7 milliseconds.
 25. A light emitter circuit tester in an electronic register for an electric energy meter of claim 24 wherein the emitter error signal from said voltage diver is fed through said comparator to said microprocessor.
 26. A light emitter circuit tester in an electronic register for an electric energy meter of claim 25 wherein said resistive impedance is a first resistor.
 27. A light emitter circuit tester in an electronic register for an electric energy meter of claim 26 wherein said voltage divider comprises a second resistor and a third resistor across the series circuit comprising said one or more light emitter diodes and said one or more transistors.
 28. A light emitter circuit tester in an electronic register for an electric energy meter of claim 27 wherein said first resistor is in series with a parallel circuit, said parallel circuit comprising said voltage divider in parallel with said one or more light emitter diodes and said one or more transistors, and the series-parallel circuit formed thereby is across a supply voltage. 